In the United States, LTE has been selected by the FCC as the technology for the Public Safety Network. In Europe, there is an ongoing discussion on the spectrum to be chosen for broadband Public Safety held by CEPT ECC WG FM PT 49. Additionally, a variety of public safety over Proximity Service (ProSe) requirements have been defined in 3GPP TR 22.803 v1.0.0 (2012-08) Technical Specification Group SA; Feasibility Study for Proximity Services (Release 12).
A public safety UE (PS UE) can operate in the public safety spectrum for public safety service and in the Mobile Network Operator (MNO) commercial spectrum for commercial services. However, only the public safety spectrum is used for public safety ProSe.
PS UEs using ProSe, communicate with each other using the Public Safety spectrum, even though they might be served by different HPLMNs. It must be noted here, that it is assumed that PS UEs may operate as commercial UEs utilizing the commercial spectrum belonging to one of the available operators, but when they operate in Public Safety mode they utilize a predefined, non-commercial, common Public Safety spectrum, that is reserved exclusively for PS UE communication.
PS UEs may either work within network coverage or outside network coverage while in both cases utilizing the public safety spectrum for the public safety services. When working outside network coverage, the PS UE cannot receive information from the network or use it for communication with other PS UEs.
A public safety UE can automatically use ProSe when network coverage is not available, or the user can manually set the UE to use ProSe discovery and communication even when network coverage is available.
In order to discover another PS UE, the PS UE listens on the radio interface at a specific time and frequency when and where other PS UEs will transmit their identities. Such identities are transmitted as proximity announcements (PAs). The PS UEs synchronize when to receive and transmit proximity announcements.
For PS UEs that are in network coverage, the network can convey synchronization information to the PS UEs. But this does not help for PS UEs that are not in network coverage. PS UEs that are outside network coverage can broadcast synchronization information themselves.
PS UEs may be set to be non-discoverable. Non-discoverable PS UEs should not transmit their identities, i.e. should not send PAs.
Assume a multitude of public safety officers (e.g. police officers, firemen, etc.) responding to an emergency situation. All the officers are equipped with PS enabled UEs. The officers must be able to discover and communicate with each other using their PS UEs, even if they are situated outside of network coverage. Even if an officer has set his/her UE to be non-discoverable from other PS UEs, he must still be able to discover the other PS UEs in vicinity.
The problem is now how to ensure that the PS UEs get synchronized when non-discoverable PS UE's are involved.